In recent years, international concern over the reduction of CO2 that is thought to be a substance causing a global warming phenomenon increases and technological innovation for shifting used energy from resource energy that emits CO2 in large quantities toward next-generation energy such as natural energy and reused thermal energy advances. As the candidates of next-generation energy technologies, technologies using natural energy such as sunlight and wind power and technologies reusing losses of primary energy such as emitted heat and vibrations caused by the use of resource energy are conceivable.
Whereas traditional resource energy is centralized energy generated mainly in massive power generation facilities, next-generation energy has the feature of being unevenly distributed in both the cases of natural energy and reused energy. In today's energy usage, energy emitted without being utilized accounts for about 60% of primary energy and the main feature is exhaust heat. Further, exhaust heat of not higher than 200° C. accounts for as much as 70% of the exhaust heat. Consequently, not only the proportion of next-generation energy in primary energy is required to increase but also technologies of reusing energy, in particular the technologies of converting the energy of exhaust heat of not higher than 200° C. into electric power, are required to improve.
When the energy of exhaust heat is intended to be used, a power generation system having a high versatility with respect to an installation mode is required because the exhaust heat is generated at various situations. As a strong candidate technology, a thermoelectric conversion technology is named.
The major part of a thermoelectric conversion technology is a thermoelectric conversion module. The thermoelectric conversion module is arranged in proximity to a heat source and generates electricity by generating temperature difference in the module. The thermoelectric conversion module takes a structure of alternately aligning an n-type thermoelectric conversion material to generate an electromotive force from a high-temperature side toward a low-temperature side along a temperature gradient and a p-type thermoelectric conversion material generating an electromotive force in the direction opposite to the n-type.
The maximum output P of a thermoelectric conversion module is determined by the product of the flow rate of the heat flowing in the module and a conversion efficiency 11 of a thermoelectric conversion material. A heat flow rate depends on a module structure suitable for a thermoelectric conversion material. Further, a conversion efficiency η depends on a figure-of-merit ZT, which is non-dimensional, of a thermoelectric conversion material. A figure-of-merit ZT is represented by ZT={S2/(κρ}T (here, S: Seebeck coefficient, ρ: electric resistivity, κ: thermal conductivity, T: temperature). In order to improve the maximum output P of a thermoelectric conversion module therefore, it is desirable to increase a Seebeck coefficient S and decrease an electric resistivity p and a thermal conductivity κ in a thermoelectric conversion material.
Meanwhile, thermoelectric conversion materials are roughly classified into a metal-based thermoelectric conversion material, a compound (semiconductor)-based thermoelectric conversion material, and an oxide-based thermoelectric conversion material. Among those thermoelectric conversion materials, as representative thermoelectric conversion materials having temperature characteristics applicable to exhaust heat recovery at not higher than 200° C., an Fe2VAl type full-Heusler alloy and a Bi—Te type semiconductor are named. An Fe2VAl type full-Heusler alloy is a metal-based thermoelectric conversion material and a Bi—Te type semiconductor is a compound-based thermoelectric conversion material. Thermoelectric conversion materials of those two types themselves can be structural materials and hence are suitable for thermoelectric conversion modules for exhaust heat recovery in a power plant, a factory, and an automobile. Problems of a Bi—Te type semiconductor however are that Te is highly toxic and the cost is high. In application for the above-mentioned exhaust heat recovery therefore, a full-Heusler alloy of a metal base such as an Fe2VAl type is more suitable than a Bi—Te type semiconductor.
With a conventional full-Heusler alloy however, the figure-of-merit ZT is only about 0.1 in the form of a bulk material that is a practical form. In exhaust heat recovery of a practically used level, a material having a figure-of-merit ZT not less than 0.1 is required.
The main reason why the figure-of-merit ZT of a full-Heusler alloy is low is that the thermal conductivity is high. As the reasons why the thermal conductivity of a full-Heusler alloy is high, named are (i) heat transfers well through the medium of electrons because the electric resistivity is low and (ii) heat transfers well through lattice vibration because the mean free path of phonons is long.
With regard to the reduction of the heat transfer derived from electrons in (i), intentional modulation is not desirable because it derives from an electronic state determining the thermoelectric conversion characteristic of a full-Heusler alloy. In contrast, with regard to the reduction of a thermal conductivity derived from lattice vibration in (ii), it can be attained by controlling the organizational structure of an alloy. It is known that a thermal conductivity can be reduced particularly by decreasing the average grain size of crystal grains in an alloy.
For example, Non-patent Literature 1 describes that grain sizes are fractionized up to about 200 nm by pulverizing and mixing a bulk material of an Fe2VAl type full-Heusler alloy with a ball mill in order to reduce a thermal conductivity and the thermal conductivity reduces to 10 W/km.
Further, Non-patent Literature 2 discloses a thermoelectric conversion material comprising an Fe2VAl type full-Heusler alloy. According to the described production conditions, the average grain size of crystal grains (hereunder referred to merely as a crystal grain size occasionally) is smaller than 200 nm in the thermoelectric conversion material.
Furthermore, Patent Literature 1 discloses a thermoelectric conversion material comprising an Fe2(TiV) (AlSi) type having a crystal structure of a full-Heusler alloy.